Compared to iron-containing enzymes, manganese systems have received relatively little attention with regard to synthetic modelling studies. The manganese aggregate in Photosystem II (PSII) is arguably one of the most important metal sites in biology as it is the source of the dioxygen required for aerobic life. The structure of the polynuclear manganese site has not been established, nor is the mechanism of the four electron water oxidation reaction which provides O2. The approach taken here to further elucidating the PSII manganese cluster involves the synthesis and characterization of small molecule analogs. The proposed research can be divided into the following specific aims: (i) further spectral, magnetic, and electrochemical characterization of the manganese oxo species prepared in the first grant period, (ii) testing of proposed mechanistic pathways for evolution of dioxygen using the model compounds already obtained, (iii) continued exploration of manganese-oxo coordination .chemistry with well designed polydentate ligands, some being derivatives of those used in the first grant period, with the goal of achieving a precise spectral match between model compound and natural system, (iv) discovery of new low nuclearity manganese species using simpler monofunctional ligands such as carboxylates, (v) development of new synthetic pathways into manganese oxo compounds, such as ligand exchange reactions using preformed binuclear species, (vi) study of the water oxidizing ,capability of new aggregates obtained in iii - v, and (vii) examination of the tendency for higher valent reactive polynuclear oxo complexes to carry out organic substrate oxidations. The last aim is . important from the standpoint that interesting reaction chemistry will surely be forthcoming regardless 1/2 of whether or not each system is capable of water oxidation.